Ramping up efforts to keep its customers’ messages safe from snooping, WhatsApp said Tuesday that it now supports end-to-end encryption for messages sent between users.

The end-to-end encryption comes thanks to a collaboration between WhatsApp and Open Whisper Systems, an open-source development company focused on secure communications.

Facebook-owned WhatsApp has more than 600 million users who log in monthly, making Open Whisper’s encryption deployment the largest ever in the area of end-to-end encrypted communication, Open Whisper said.

The encryption is on by default. It’s only available for Android right now, though the companies are working to roll out support for other platforms.

End-to-end encryption has gained attention following the disclosures about government surveillance last year by former NSA contractor Edward Snowden. Meanwhile, the flood of cyber attacks targeting retailers and Internet companies alike have highlighted the need for better data security.

Edward Snowden himself has called end-to-end encryption the best possible form of encryption, because it keeps people’s data encrypted even while it’s on company servers. The data, in theory, can only be decrypted on people’s personal devices. That means outside groups must target individuals’ machines if they want to access the data.

Some other mainstream services like Google have released products to facilitate end-to-end encryption. And along with Apple, Google’s also working to make encryption the default on smartphones.

But end-to-end encryption still is primarily offered by lesser known companies that don’t rely on people’s data for advertising.

WhatsApp’s end-to-end encryption uses Whisper’s TextSecure protocol, which encrypts text messages over the air and on people’s phones. The integration has been in the works for the past six months, and the lessons learned along the way are likely make their way into other deployments, Whisper said.

Some WhatsApp users, however, still have concerns over the security of their messages, particularly in light of the company’s new owner. Facebook’s acquisition of WhatsApp closed last month.

“How can we trust WhatsApp to actually securely have the TextSecure code implemented, seeing its proprietary license?” one person wrote on Whisper’s site following the announcement. “What is preventing them from leaking plaintext versions of the messages to Facebook?”